1. Field of the Invention
This invention relates to a process for producing polyphenylene oxides.
2. Description of the Prior Art
Polyphenylene oxides and numerous processes for producing them are known. Cf., for example, Buehler "Spezialplaste," Adademieverlag, Berlin, 1978. In industry, one successful process entails the oxidative coupling of diorthosubstituted phenols in the presence of oxygen by copper-amine complexes.
In the process of DBP No. 15 70 683, a catalyst of a copper (II)-amine salt complex of the formula Cu(OX)X.RR'--NH is used, wherein X is an acid anion, R and R' are alkyl radicals and R' can also be hydrogen. This catalyst assumes the role of an oxygen carrier. The active complex is produced by mixing a copper (II) salt with copper (II) hydroxide, or by adding a base to a copper (II) salt, in the presence of a primary or secondary aliphatic amine. Alternatively, a similar complex can also be formed during polycondensation, by reacting a copper (I) salt and an amine of the formula RR'--NH, and oxidizing the resulting complex. However, complexes of these amines are not active with non-basic copper (II) salts or copper (II) hydroxide. The reaction is usually quenched by the addition of an inorganic acid or a base. Thus, DE-PS No. 15 70 683 disclosed the first technically usable process for the production of polyphenylene oxides (PPO).
Unfortunately, drawbacks soon become apparent in the large-scale application of this process. For example, high catalyst concentrations are required to reach an acceptable reaction rate and degree of polymerization. Moreover, copper-amine complexes with higher catalytic activity require more costly components, for example, anhydrous copper salts, special amines, and additional metal bromides. For this reason, it is always harder to separate the PPO from tne amine and the residual catalyst ingredients. An expensive recycling is required to recover the amines.
Further, the use of oxygen as an oxidizing agent involves the risk of fire and explosion, in addition to tne higher costs in comparison with the use of air. However, many of the processes of tne prior art involve the use of oxygen; in others the reaction rate drops drastically by using air, so that the PPO production becomes uneconomical.
Also, the formation of colored by-products, such as diphenoquinone, is very bothersome. Complete separation from the PPO is difficult.
The most important processes are listed below:
DE-PS No. 16 45 515 (U.S. Pat. No. 3,384,619) describes a process for polycondensation of phenols to high molecular weight polyphenylene oxides, in which a catalyst containing a tertiary amine and a nonbasic copper(II) halide is used. The catalyst concentration in this process is unusually high. With tne use of 9 parts of amine per one part of phenol, the entire process is economically unattractive.
According to DE-PS No. 20 11 709 (U.S. Pat. No. 3,639,656) a complex of a primary or secondary amine with an anhydrous, nonbasic copper(II) salt is used. In this way, the amount of catalyst is reduced to 0.01 mole of copper and 0.15 mole of amine per mole of phenol. It is especially important that the copper salts used are anhydrous.
DE-PS No. 20 11 711 (U.S. Pat. No. 3,661,848) describes a process for the production of PPO in the presence of a nonbasic copper(II) salt, a straight-chain aliphatic amine and a low molecular alcohol in an amount of less tnan 5, preferably 0.5 to 3 percent by volume. Oxygen is used as the oxidizing agent.
According to the process of DE-PS No. 22 17 161 (US-PS 3 989 671) a catalyst of certain copper(II) salts, tertiary diamines and an iodine compound is used. While it is possible with this process to obtain PPO having a viscosity of 1.3 dl/g, the use of the costly iodine compound is not justified.
DE-PS No. 22 28 071 (U.S. Pat. No. 3,733,299) describes a catalyst containing an alkaline-earth metal bromide besides a mono- or bivalent copper salt and an amine. Oxygen is required as the oxidizing agent. The use of metal bromides has the drawback that processing is complicated by the presence of the additional metal ions.
DE-OS No. 24 60 326 (U.S. Pat. No. 3,900,445) describes a catalyst of an anhydrous copper(I) and copper(II) salt and a primary, secondary or tertiary amine. In this way, the use of oxygen is improved, although the danger of fire and explosion in the reactor remains.
DE-OS No. 25 05 328 (U.S. Pat. No. 4,028,341) describes a catalyst of a copper(I) or copper(II) salt, a bromide ion and at least a secondary alkylenediamine and a tertiary monoamine.
In addition, the catalyst of DE-OS No. 27 38 889 (U.S. Pat. No. 4,092,294) consists of a secondary monoamine. Apart from the problem of the availability of this amine, there is the additional problem of having to process a complicated amine mixture. A further drawback of both of these processes is that the amine components, in keeping with their respective volatility, are carried off from the reaction in varying degrees by the gas introduced. In this way the activity of the catalyst during the reaction can change.
To remedy this drawback it is proposed in DE-OS No. 29 13 204 (U.S. Pat. No. 4,211,857) to circulate the gas, which has been introduced in the reaction system, in a closed system, then separating the entrained amine by cooling and replacing the used oxygen continuously.
DE-OS No. 30 35 599 mentions as substantial drawbacks of the known processes the accumulation of dangerous explosive gas mixtures in the reaction chamber and formation of the by-product diphenoquinone, which is hard to separate from the PPO (cf. DE-OS No. 21 34 095=U.S. Pat. No. 3,637,593). However, as before, the examples teach the use of pure oxygen and anhydrous copper salts. Therefore, this solution is not convincing.
German Patent Application Nos. P 32 24 692.7 and 32 24 691.9 disclose PPO production processes in which morpholinium bromide or polyhydric alcohols and aqueous alkaline-earth hydroxide solutions are used as activator combinations in determined molar ratios. It is indicated that the J values, which are used as the measurement of the rate of polymerization, are considerably improved in comparison with the processes in which only one of the components morpholinium bromide, polyhydric alcohols or alkali hydroxides is used as activator. The high amounts of amine, required by this process, are disadvantageous.
Hence, all of these processes show the difficulties involved in the oxidative coupling of diorthosubstituted phenols to produce polyphenylene oxides. Accordingiy, a need continues to exist for a process for oxidatively coupling diorthosubstituted phenols which produces polyphenylene oxides in a safe and simple manner.